Frequently, the repair of such double-shell vessels is necessitated by the corrosive properties of one of the fluids flowing therethrough, the fluid causing the occurrence of holes in the inner shell. Such corrosion has heretofore necessitated the removal of the exchanger from service in order to repair the inner shell. Employing the apparatus and method of the present invention in those instances where fluids under relatively low pressures are concerned, this invention allows repairs to be made under operating conditions with little or no loss of production.